Combination pH Electrodes:
A combination pH electrode is one in which the sensing and reference half-cells are integrated into a single assembly, usually a cylindrical probe for dipping into a test solution. For the purposes of the following descriptions and explanations, the standard potential can be defined as the potential in any certified standard. Here, the potential in pH 7.00 buffer as test solution will be used. The potential, or strictly speaking “potential difference”, of a combination pH electrode in a solution is actually an algebraic sum of at least five potential differences, henceforth referred to simply as potentials. These potentials are defined as follows:
PotentialDescriptionE1Potential between inner metal element of pH half-cell andelectrolyte in the glass pH bulb.Depends on identity of metal and composition of pH bulbelectrolyte.E2Potential between pH bulb electrolyte and glass of pH bulb.Depends on pH of pH bulb electrolyte.E3Potential between glass of bulb and test solution.Depends on pH of test solution.E4Potential at junction of test solution and inner reference half-cell electrolyte (commonly known as junction or liquid-junction potential)Depends on compositions of test solution and referenceelectrolyte.E5Potential between reference half-cell electrolyte and innermetal element of reference half-cell.Depends on identity of metal and composition if referencehalf-cell electrolyte.Symmetrical Cells:
In order for the standard potential to be stable, all five of these potentials must be stable. Usually, electrode designers strive toward a system where the standard potential, i.e., the algebraic sum of these five potentials, equals zero in a test solution having a pH of 7.00. Since pH 7.00 is considered neutral, a standard potential of zero is convenient for the designers of meters that measure the potential, and there are advantages with regard to the effect of temperature on the potential that will become clear as this description advances.
Unless otherwise noted, all electrodes discussed herein will be so-called “symmetrical cells”, those where a design goal is a potential close to zero millivolts at pH 7.00.
In a symmetrical cell, the metal elements in the pH and reference half-cells are the same and the potential-determining components of the pH bulb electrolyte and reference electrolyte are the same. Potentials E1 and E5 are thus equal in magnitude but opposite in sign and cancel. Similarly, by adjusting the pH of the pH bulb electrolyte to 7.00, potentials E2 and E3 become equal in magnitude and opposite in sign and also cancel, unless a small difference known as the asymmetry potential is present as discussed below. E4, the junction potential, cannot be entirely eliminated but can be minimized if a high concentration of equitransferent salt is included in the reference electrolyte. This is also discussed further below. Thus, in symmetrical cells, the problem of maintaining as stable a standard potential as possible over time, so as minimize the need to recalibrate frequently, is often equivalent to maintaining a standard potential of zero.
Asymmetry and Junction Potentials:
A brief discussion of asymmetry and junction potentials is in order. The asymmetry potential across the pH-sensitive glass is any non-zero potential difference that exists when the pH values of the solutions on both sides of the glass are equal. An experiment can be designed to measure asymmetry potentials. A solution can be split into two portions, isolated by pH sensitive glass, and the potential between two identical reference electrodes placed in the two portions of solution can be measured to yield the asymmetry potential. However, in practice, measurement of the asymmetry potential in a combination pH probe is not always possible. For example, if a symmetrical cell is devised, in which the pH bulb electrolyte is adjusted to 7.00 and the potential of the probe is measured in pH 7.00 buffer, a non-zero potential might be ascribed to asymmetry potential or to junction potential or both. The pH 7.00 buffer does not contain the components required to establish potentials E1 and E5, so the pH bulb electrolyte and the reference electrolyte must necessarily have a different composition than 7.00 buffer. This creates two inevitable uncertainties.
First, the junction potential between the reference electrolyte and the 7.00 buffer will be unknown. Junction potentials can be estimated theoretically, but cannot in most cases be measured because another uncertain potential is always introduced into any cell that is conceived to attempt a junction potential measurement. This is a fundamental dilemma of electrochemical cells (Bates, R. G., Determination of pH: Theory and Practice, 2nd Ed., John Wiley & Sons, New York, p. 33 and elsewhere).
Second, an uncertainty in the pH value of the bulb electrolyte is always present. This is because its pH was adjusted through comparison to a 7.00 buffer with some other pH cell, the junction potential of which cannot be assumed to be the same in pH 7.00 buffer and in the pH bulb electrolyte. Some uncertainty may even exist in the perfect cancellation of potentials E1 and E5 because the presence of a buffer substance in the pH bulb electrolyte, that may not be present in the reference electrolyte, may alter the activity of the components that determine E1 and E5. Often, this problem is overcome by saturating each electrolyte with these potential-determining components such that their activity is fixed at the saturation level.
Temperature Effects:
It is not within the scope of this specification to go into great detail about temperature effects on pH cells, but brief mention is appropriate since temperature will play a role in parts of the discussion. It's clear why a symmetrical cell is desirable when considering the effect of temperature. To the extent that potentials E1 and E5, as well E2 and E3, have essentially the same half-cell composition, temperature changes will affect them equally and cancel. This leaves E4, the junction potential, as potentially contributing to a change in standard potential as temperature changes. Suffice to say at this stage that the best approach to resolving the issue of temperature effect on E4 is to design cells with smallest junction potential achievable.